The present invention relates to timing of teeth in gear assemblies. In particular, the present invention is directed to timing of teeth on gears assembled in a cluster such as are used in epicyclic gear systems.
Epicyclic gear systems comprise a plurality of planet gears disposed within a ring gear to surround a central sun gear. In typical configurations, the sun gear is used as an input while the ring gear is used as an output, with the planet gears providing a gear reduction and directional reversal from the input to output. Epicyclic gear systems can, however, have different configurations with different input sources and output sources as well. Gear teeth on the planet gears, therefore, must align with teeth on both the sun gear and the ring gear. More specifically, gear tooth position about the gear axis on each planet gear must be the same, or within a tolerance band, such that timing with the sun gear and ring gear for each planet gear is the same. Consistency between tooth positions reduces uneven loading on the planet gears. Disparate loading of one of the planet gears has the effect of removing the other planet gears from the system or otherwise reducing loading on those gears. The loaded gear then suffers increased stress and wear and the epicyclic gear system operates inefficiently.
The effect of gear tooth timing in epicyclic gear systems is compounded when planet gear clusters are used. Planet gear clusters typically comprise a large diameter planet gear that engages the sun gear, and a small diameter planet gear, which is axially aligned with and fixed to the large diameter planet gear, that engages with the ring gear. As such, it becomes necessary to make sure that the large diameter planet gear teeth consistently engage with the sun gear, and that the small diameter planet gear teeth consistently engage with the ring gear, while ensuring that the gear teeth on the large diameter and small diameter planet gears are properly aligned, or timed, with each other.
Conventional methods from timing gear cluster teeth involve aligning the centers of the small diameter gear teeth with the centers of the large diameter gear teeth. In such a scheme, the pitch points along the sides of the gear teeth are timed adequately when each gear tooth in the large diameter and small diameter planet gear is the same, such as at the nominally dimensioned width or thickness between flanks. However, when the gear tooth on the large diameter planet gear and the gear tooth on the small diameter planet gear vary from the nominal dimension to some other value, even within acceptable tolerances, alignment of the pitch points will vary from pair to pair. Significant variation in pitch alignment from pair to pair can lead to uneven loading of the planet gear clusters when assembled within an epicyclic gear system.